Testing a USB Device

The USB Virtual Bus framework is a helpful framework to connect a USB Function driver to a USB device driver for testing.

The following files are involved in testing a USB Device driver. All three of these files should be stored in the same directory:

  • {driver}.cc: the USB device driver that will be tested.
  • {driver}-function.cc: A USB function driver that fakes the underlying USB device hardware.
  • {driver}-test.cc: The test program, which sets up the driver and runs tests.

The usb-virtual-bus connects the USB Peripheral bus to the USB bus, as seen in the below graphic:

usb-peripheral-bus -> {Your usb-function driver}
usb-bus -> {Your usb-driver}

Write a USB-function driver

A usb-function driver makes the current host appear like a peripheral USB device. For example, the USB mass storage (ums) function device allows the host to appear as a block device when it is plugged into another machine. If it makes sense for your host to have a USB function driver for your class of device, then a real usb-function driver should be created. Otherwise, writing a usb-function that fakes the hardware of a usb device is the easiest way to test your usb driver.

The usb-virtual-bus connects your usb-function driver to the actual USB device driver you are trying to test. This allows the device driver to be run in a test mode with no modifications to the device driver.

Examples of usb-function drivers:

The usb-function driver needs to implement the UsbFunctionInterface banjo interface. These are the functions that are called from the usb-virtual-bus library as it sets up the driver in the USB stack.

A usb-function driver binds on top of the UsbFunction protocol. These are the calls that allow the function driver to allocate endpoints, register interface callbacks, queue USB requests, and more.

Bind rules

The usb-function driver needs to bind to the ZX_PROTOCOL_USB_FUNCTION protocol. There can be additional bind rules for the USB class, USB subclass, and USB protocol.

This example shows a bind rule where {} represents an area that should be replaced with your information:

using fuchsia.usb;

fuchsia.BIND_USB_CLASS == {usb_class}
fuchsia.BIND_USB_SUBCLASS == {usb_subclass}
fuchsia.BIND_USB_PROTOCOL == {usb_protocol}

Writing the usb-virtual-bus test

The test should be written using the usb virtual bus launcher library.

The first thing the test launches is the usb-function driver described in Write a USB-function driver. You can launch this test by adding the bind rules to a usb_peripheral::FunctionDescriptor and using the SetupPeripheralDevice() function. For example:

// Set up your USB Device Descriptor.
usb_peripheral::DeviceDescriptor device_desc = {};

/ Set up your USB Function descriptors.
std::vector<usb_peripheral::FunctionDescriptor> function_descs;
usb_peripheral::FunctionDescriptor function_desc = {
    .interface_class = {usb_class},
    .interface_subclass = {usb_subclass},
    .interface_protocol = {usb_protocol},

ASSERT_NO_FATAL_FAILURE(SetupPeripheralDevice(device_desc, std::move(function_descs)));

Once the SetupPeripheralDevice function has succeeded, the usb-function driver binds.

The USB virtual bus connects the function driver into the system, and then the real device driver binds. Your test can then connect to the USB device driver through devfs. Binding happens asynchronously, so you have to wait for the driver to be detected by devfs. The east way to watch for a file is the fdio_watch_directory function.

Now that you've connected to your device, FIDL calls can be made normally. Your driver handles those FIDL calls and makes calls to your USB function driver as if it were real hardware.